New Mexico Geological Society Annual Spring Meeting — Abstracts

In 1966, Zeller (NMBMMR Memoir 16) described the Borrego section of the PennsylvanianPermian Horquilla Fm in the Big Hatchet Mountains (SE¼ sec. 27, T31S, R15W, Hidalgo County). He interpreted the Virgilian-Wolfcampian interval here as a single, homoclinal section about 500 m thick divided into 5 units: (1) lower, limestone-dominated unit, ~120 m thick, late Virgilian-early Wolfcampian; (2) light gray “basinal” shale ~105 m thick; (3) middle limestone, ~60 m thick, early-middle Wolfcampian; (4) light gray “basinal” shale ~102 m thick; and (5) upper limestone, ~90 m thick, middle Wolfcampian. In contrast, Drewes (1991, USGS Map I-2144) mapped shingled thrust faults in which units 1-2 are Horquilla Fm (1) depositionally overlain by Earp Fm (2), separated by a thrust fault from units 3-4, which are Horquilla-Earp again, separated from another thrust fault from unit 5, which is Horquilla. Our study of the Borrego section indicates that Zeller’s identification of a single, homoclinal section is correct, though we differ from Zeller in identifying units 2 and 4 as very shallow marine facies of the Horquilla Fm (mostly crossbedded calcarenites, thinly laminated siltstones and grainstones), not a basinal facies. Several observations refute the thrust fault interpretation of the Borrego section: (1) regionally, the Horquilla-Earp contact is late Wolfcampian, so if unit 2 is Earp deposited on Horquilla, then this contact is locally early Wolfcampian, and if unit 4 is Earp deposited on Horquilla, then the contact is middle Wolfcampian, both highly unlikely; (2) fusulinids from units 1, 3 and 5 are in correct temporal order; (3) all beds throughout the section dip ~30-35º to S50ºE; and (4) there is no structural evidence of thrust faults at the bases of units 3 and 5. Therefore, we reject the presence of shingled thrust faults in the Borrego section of the Horquilla Fm.

Unit 1 and the lower part of unit 2 cross the Virgilian-Wolfcampian boundary and contain conodont faunas of the following ages: (1) bed 6, middle Virgilian, Lecompton cyclothem (=middle of Shawnee Group); (2) bed 38, late Virgilian, Brownville level (=top of Wabaunsee Group, just below base of Newwellian) (3) bed 44 (top of carbonate section), lower Wolfcampian (Newwellian?) (=Admire Group); (4) beds 50, 54, carbonates in base of unit 2, Wolfcampian (Newwellian) (=Admire Group to base of Council Grove Group). All conodont faunas are dominated by Streptognathodus, associated with fewer Adetognathus--a typical normal marine, shallow water, shelf association. There is no conodont evidence of a “basinal” setting. The basal 7 m of unit 1 is a clastic sequence composed of sandy conglomerate, sandstone and siltstone of fluvial origin. Above a 4.8-m thick covered interval, a 105-m-thick succession of bedded and massive, fossiliferous limestone is exposed. The limestones are mostly bioclastic wackestone, subordinate thin beds of fusulinid wackestone/packstone and massive algal limestone. Solitary corals, brachiopods, gastropods and crinoids occur throughout the section. The thin covered intervals probably are marly shale and shale. Lithofacies and fossil content indicate that the limestones formed in a shallow marine, shelf environment.